Of various known anticancer agents, cisdiaminedichloroplatinum (II) (Cisplatin.TM.) now is preferred because of its broad antitumor effects. However, Cisplatin.TM. also has strong toxicity causing side effects, such as nephrotoxicity, disturbances of hematopoietic organs, and disturbances of auditory nerves. Cis-diamine(1,1cyclobutanedicarboxylato)platinum (II) (Carboplatin.TM.) was developed for the purpose of reducing the side effects of Cisplatin without weakening its antitumor activity. However, Carboplatin has been reported to still have strong side effects such as nausea, vomiting, and anorexia, as is usual with anticancer agents.
Presently, such platinum-containing anticancer agents are administered alone. There are no cases reported in which such a platinum-containing anticancer agent is supported on a high-molecular weight compound so as to suppress the side effects.
On the other hand, in order to inhibit manifestation of unfavorable side effects of a drug, it has been attempted to minimize the concentration of the drug. One of the approaches is to incorporate a drug into a synthetic high-molecular weight polymer so that the release of the drug may be sustained by the membrane function of the synthetic high-molecular weight polymer. The thus sustained release usually has a duration of from 2 to 3 days. However, use of a synthetic high polymer as a carrier for a drug carries the risk of side effects arising from accumulation of the synthetic high polymer in the living body.
To avoid the risk, use of chitin in an acetylated form, a biodegradable natural high-molecular weight compound, as a carrier for a drug is attracting attention (see Ogawa Kozo, et al., Chitin-Chitosan no Oyo (edited by Chitin-Chitosan Kenkyukai) (1990)). The inventors of the present invention previously proposed a process for preparing a slow-releasing composition releasing a drug for a prolonged duration by using chitin in an acetylated form obtained from crustaceans, e.g., crabs and lobsters, in a molded form as disclosed in JP-A-61-268616 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). The proposed slow-releasing composition, however, was still unsatisfactory in practice, though successful in reducing the residual carrier substance in a living body and extending the duration of drug release.